Ferromagnetic resonance damping in garnets: comparison between saturated and unsaturated states

The influence of magnetic state (saturated or unsaturated states) on the ferromagnetic resonance damping in relaxing rare earth substituted Y_1-xRE_xGd₂Fe₅O ₁₂ garnets (with RE=Dy Ho, Tb and x=0;0.15;0.3;0.6) has been studied. The damping parameter a, as defined by Landau-Lifshitz equation, has been determined either by FMR at 10 GHz (α_sat for saturated state) or by measurement of the frequency dependence of the complex initial permeability in the 0.1 GHz-10 GHz band (α_unsat for unsaturated state). In the saturated state the introduction of relaxing rare earth is found to increase significantly α_sat values by more than one order of magnitude. On the contrary, although α_unsat values also increase with rare earth concentration they stay in the same order of magnitude. It appears that α_unsat is controlled either by the magnetic disorder introduced via the domain and domain wall structure or by the rare earth concentration